Dental restorations, such as crowns, bridges, implants, dentures and tooth replacements, can be produced by a number of different manufacturing processes using various materials. In recent years, developments in materials and manufacturing processes have focussed on the production of dental restorations based on ceramic materials. Generally, ceramic dental restorations comprise a base section or support framework which provides the necessary strength, and additionally a facing section or veneer is applied which provides the necessary aesthetic appearance. More recently, the use of monolithic materials in dedicated monolithic zirconia dental restorations has become more commonplace. The advantage of monolithic restorations, that is to say restorations formed of one material instead of a combined framework and veneer structure, is that the dentist or clinician is able to remove less material from the natural tooth because the monolithic zirconia restoration can have a thinner wall thickness compared to other restorations.
The most important properties of ceramic restorations, especially dental zirconia restorations, are the ceramic material's fracture strength and fracture toughness. This is the ability of the material to resist propagation of an internal crack or fracture, and is one of the most important indications of the material's clinical reliability. However, a design compromise is sometimes needed, depending upon the particular clinical situation. In particular, a design needing higher fracture strength limits the aesthetics and the functional parameters of the dental parts. For example, a large connector cross section in an anterior bridge influences the final design and veneering, and limits the natural look of the restoration.
Ceramic restorations are often manufactured using automated CAD/CAM (computer-aided design/computer-aided manufacturing) technology, which typically include a 3D scanning device, a milling machine and a sinter furnace, all of which are controlled by appropriate computer software. The workflow starts with the dentist or clinician taking an impression from the patient and sending this impression to a dental technician. The dental technician builds a plaster model based on the impression and scans this model with the 3D scanner. Based on the scanned data, the dental technician designs a new dental restoration 3D model. This 3D model is the basis for the CAD/CAM process. The dental restoration 3D model is also scaled accordingly to take into account the predicted shrinkage that occurs during the sintering phase.
Typically, the CAM module of prior art systems has, depending on the dental indication, standardised milling strategies. That is to say, for example, every bridge will be machined by calculated milling steps from rough milling to dress/fine milling until the dental part is completed. These steps and milling strategies are calculated individually in the CAM module for each of the dental parts. After the calculation of these steps, the CAM module translates this milling sequence to machine code and this is sent to the particular milling machine for the milling process to commence. After the zirconia restoration is milled, a subsequent sintering stage is then needed to achieve the final shape and mechanical properties.
3M EPSE Lava™ is such a prior art CAD/CAM technology for dental restorations on a zirconium oxide base.
A further prior art technique for milling ceramic dental restorations can be found in WO 2006/105944 A1 which discloses a grinding and polishing tool, and a method for machining ceramic materials, including zirconium oxide.
However, there is still need for improvements, especially with respect to the enhanced requirements of modern dental materials and clinical indications. Significantly increasing the fracture strength of milled zirconia parts, by producing a smoother surface with a reduced number of flaws or surface defects, will considerably expand the clinical indication spectrum of digital restorative dentistry for both monolithic restorations and combined framework and veneer structures. For example, it is desirable to be able to produce a highly aesthetic low cross section anterior bridge restoration, which has been difficult to achieve using known CAD/CAM technology.
Moreover, there is a need for a process which allows the manufacturing of ceramic dental restorations in a simple, timely and efficient manner.